How can I avoid seeing the inside faces of a semitransparent object in the game engine?
In the material editor (cycles) add a geometry node and look at the "Backfacing" output. Plug it into the fac of a new mix shader where one input is what you already have and the other is a completely white Transparent shader. You haven't uploaded a .blend file so I can't know what  exactly you are doing but I hope this helps.